Observations of interstellar absorption lines toward both members of
resolvable binary star systems can provide a sensitive probe of
small-scale structure in the diffuse ISM\@. We have recently obtained
very high resolution (\Deltav \approx 0.3 km s^-1)
observations of the interstellar Na I D lines toward several
binaries with the Ultra-High Resolution Facility (UHRF) on the
3.9 m Anglo-Australian Telescope. In the case of \mu Cru, which
consists of two B stars separated by 38\farcs8 at a distance of
170 pc, these observations reveal line strength variations in
at least four velocity components indicative of ISM structure on
scales less than the projected binary separation of 6600 AU (0.03 pc).
The most striking of these variations involves a Na I
component seen at a heliocentric velocity of -9 km s^-1
toward \mu^1 Cru but not at all toward \mu^2 Cru.
This component has an appreciable column density
(N(Na I) = 7 x 10^10 cm^-2) and it clearly exhibits
hyperfine splitting with a Na I line width
(FWHM \approx 0.7 km s^-1) indicative of a cloud
temperature below 200 K\@. We have also obtained UHRF
spectra of the interstellar Ca II K line toward \mu Cru
which reveal a N(Na I)/N(Ca II) ratio (\approx7) for the
-9 km s^-1 component that is suggestive of some dust
depletion but is much lower than that of typical diffuse
clouds. The other obvious profile differences in this sightline
include a pair of cold components near +4 km s^-1
that are much more pronounced toward \mu^2 Cru than
\mu^1 Cru. Given the remarkable variations in multiple
velocity components toward \mu Cru, it should not be too
surprising that our high resolution observations of Na I
toward other binaries reveal that such small-scale structure
in the cold ISM is common. We will discuss the
implications of these results for a better microphysical
understanding of diffuse interstellar clouds.